Multiple TEXTURE Displacements (mapping) in one scene
(I'm not totally clear on you question but I'll give it a shot.)

You can combine several displacement textures one using Mix RGB nodes.

Let's say you have a situation like this where I'm combining two textures used as displacement:



Now I want to add some voronoi noise to the whole thing, so I would just combine the bump map with a mix RGB node like this:



But as noted in one of the comments using true displacement on the material nodes is still at experimental stages and might yield unpredictable results. It might be better to combine multiple displacement modifiers, and maybe use an empty to control the coordinates for the deformation.